The application of C N C systems to machine tools in particular has significantly advanced the machining of shapes, particularly shapes which are not of circular or straight line forms which are normally quickly and accurately accomplished using conventional machine tools.
The simplest method of applying a C N C system is to use incremental drives, such as stepping motors, which rotate the leadscrews of a machine tool by fixed increments corresponding to the outward pulses from a computer.
Industry now demands both high dimensional resolution (typically 10.sup.-4 in (2.54 .mu.m)), and high machine speed (typically 200-300 in/min (5-7.62 m/min)), thus requiring drive steps of 10.sup.-4 in (2.54 .mu.m) to obtain the desired dimensional resolution and an output to the stepping motors from the computer control system of up to 5.times.10.sup.4 pulses per second. Commonly, contour interpolation programmes for two and three dimensional motion contain from about 100 to several hundred instructions. With such requirements a relatively inexpensive computer, with an average execution time of 2 .mu.sec per instruction has a pulse output or drive pulse to the stepping motors of only 10% of the desired rate or less.
Therefore, with conventional drives and interpolating methods, the machine speed can only be increased to that demanded by industry by increasing the size of the drive steps executed by the stepping motors which results in a reduction of dimensional resolution thus failing to meet industry's other demand.
Attention has been given to developing more complex, and therefore expensive, control systems and/or incremental drives, to allow for greater travel per drive pulse whilst keeping the surface quality to the required standard. Such systems are disclosed in a paper shortly to be published by CIRP and entitled "Digital Controls and Feed Drives State-of-the-Art and New Developments" by J. G. Bollinger, G. Stute, H. Van Brussel and J. Dinsdale. Another example of the complexity of such known systems capable of coping with the demands of industry in relation to high dimensional resolution and high machine speed, is disclosed in U.S. Pat. No. 3,969,615 (Bowers et al).
It is an object of the present invention to provide a method of forming contours, and a control system adapted for association with the contour forming apparatus, which, in the particular application to machine tools, will achieve the demands of industry in relation to both high dimensional resolution and high machine speed, but which is inexpensive compared with known control systems capable of achieving such demands.